81 lines
1.6 KiB
Java
81 lines
1.6 KiB
Java
import java.util.*;
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import java.util.function.IntFunction;
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public class CycleDetection {
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public static int[] floyd(IntFunction<Integer> f, int x0) {
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int tortoise = f.apply(x0);
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int hare = f.apply(f.apply(x0));
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while (tortoise != hare) {
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tortoise = f.apply(tortoise);
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hare = f.apply(f.apply(hare));
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}
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int startPos = 0;
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tortoise = x0;
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while (tortoise != hare) {
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tortoise = f.apply(tortoise);
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hare = f.apply(hare);
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++startPos;
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}
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int length = 1;
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hare = f.apply(tortoise);
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while (tortoise != hare) {
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hare = f.apply(hare);
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++length;
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}
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return new int[]{startPos, length};
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}
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public static int[] brent(IntFunction<Integer> f, int x0) {
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int power = 1;
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int length = 1;
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int tortoise = x0;
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int hare = f.apply(x0);
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while (tortoise != hare) {
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if (power == length) {
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tortoise = hare;
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power *= 2;
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length = 0;
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}
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hare = f.apply(hare);
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++length;
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}
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int startPos = 0;
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tortoise = x0;
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hare = x0;
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for (int i = 0; i < length; i++)
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hare = f.apply(hare);
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while (tortoise != hare) {
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tortoise = f.apply(tortoise);
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hare = f.apply(hare);
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++startPos;
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}
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return new int[]{startPos, length};
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}
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// random test
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public static void main(String[] args) {
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Random rnd = new Random(1);
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for (int step = 0; step < 100_000; step++) {
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int a = rnd.nextInt(100);
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int b = rnd.nextInt(100);
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int c = rnd.nextInt(100) + 1;
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IntFunction<Integer> f = x -> (a * x * x + b) % c;
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int[] floyd = floyd(f, 0);
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int[] brent = brent(f, 0);
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if (!Arrays.equals(floyd, brent))
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throw new RuntimeException();
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}
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System.out.println(floyd(x -> (41 * x + 1) % 1000_000, 0)[1]);
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}
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}
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